The present invention relates to a method for forming an interlayer insulating film which electrically insulates a wiring from other wirings in a semiconductor device, more particularly to a method for forming an interlayer insulating film showing a low dielectric constant, which reduces the electric capacitance between the wirings.
Recently, high integration and high densification of semiconductor devices such as LSI have progressed by reducing the spacing between the wirings in the semiconductor device. As the spacing between the wirings becomes narrower, electrical capacitance between the wirings becomes larger and processing speed of the semiconductor device becomes slower. To overcome such a problem, a film showing a low dielectric constant must be used as an interlayer insulating film between the wirings, and development of such an interlayer insulating film showing a low dielectric constant is an urgent need.
The interlayer insulating film that has been used most widely is an SiO2 film, and its relative dielectric constant is 4.0. Interlayer insulating films showing a lower relative dielectric constant than that of an SiO2 film include an FSG film obtained by adding F (fluorine) to SiO2, a HSQ (Hydrogen Silsequioxiane) film of an inorganic SOG (Spin On Glass), and an organic polymer film. Next, these films will be briefly described.
FSG Film
There are several methods to form the FSG film, and a formation method using a helicon wave plasma CVD apparatus will be described in this specification (see Semiconductor World (Monthly), February 1996, p.82 to p.85). In this method, SiH4, O2, Ar and SiF4 are used as source gases. Among these source gases, SiF4 is used for adding fluorine (F) to SiO2. Process conditions using these source gases are as follows.
gas flow rate: SiF4+SiH4=70 (sccm)
SiF4/SiH4=1.0
O2/(SiF4+SiH4)=1.0
Ar=70 (sccm)
power of helicon wave plasma: 2500 (W)
bias power: 2000 (W)
temperature: 400 (xc2x0 C.)
The relative dielectric constant of the FSG film formed under these process conditions is at least about 3.1, which value is smaller than the 4.0 relative dielectric constant of an SiO2 film most generally used.
HSQ Film
HSQ is inorganic polymer of HSiO1.5, and contains hydrogen, thus showing a low relative dielectric constant of about 2.9.
Other Films
In addition to the FSG film and HSQ film, films of Teflon group and other organic polymers have been studied. However, these films showing low relative dielectric constants have the following problems.
HSQ Film
The HSQ film shows poor stability in a thermal annealing process and, when oxygen, nitrogen and the like touch the surface of the film, its dielectric constant decreases. A graph showing the relationship between the time of annealing the HSQ film and its dielectric constant is shown in FIG. 1.
Other Films
The films of the Teflon group show poor adhesion to a SiO2 film, and they tend to peel off from the SiO2 film during Chemical Mechanical Polishing (CMP).
Since the films of the organic polymer group show an etching rate approximately equal to that of resist for a mask member, it is difficult to etch them. These organic polymer films have a problem that when this resist is subjected to O2 down-flow-ashing, the film of the organic polymer formed under the resist is damaged by oxygen O2, and the dielectric constant of the film is increased.
The object of the present invention is to provide a method for forming a new interlayer insulating film showing a low dielectric constant, which is different from the above-described films, and a semiconductor device fabricated by use of this method.
According to the method for forming an interlayer insulating film of the present invention, a side portion of a patterned metal film is selectively etched. Thus, overhung portions of a first insulating film on the metal film are formed, and then a second insulating film is formed over the entire structure.
When the spacing between wirings is narrowed, a gap between overhung portions formed on the wirings is also narrowed. For this reason, a source gas for forming the second insulating film will hardly enter a concave portion defined by side surfaces of two adjacent wirings and the a surface of a substrate. Thus, the second insulating film is not sufficiently formed in this concave portion, and the result is a cavity in the concave portion.
The dielectric constant of the cavity formed in such manner is apparently smaller than that of the second insulating film. Accordingly, the dielectric constant of the narrow spacing between the wirings become smaller compared to the case where that spacing is fully filled with the second insulating film.
In the method for forming an interlayer insulating film of the present invention, a SiN film is used as a first insulating film. The SiN film is formed by a plasma enhanced chemical vapor deposition method using SiH4 and NH3 as source gases. The inventors of the present application found out that the overhung portion of the SiN film can be easily formed by setting a flow rate ratio SiH4/NH3 to be larger than 1.0. The reason for this is that the SiN film formed in this manner is hardly affected during etching of the patterned metal film.
Instead of the above SiN film, a Diamond Like Carbon (DLC) film can be used as a first insulating film. According to measurement carried out by the inventors of the present application, the dielectric constant of the DLC film containing many air bubbles therein is about 2.4 to 2.7, which is smaller than the dielectric constant (about 7.0) of the SiN film. Accordingly, the use of the DLC film enables the dielectric constant of the entire film to be smaller than that of a SiN film used as a first insulating film. Furthermore, compared with the SiN film, the DLC film is more stable physically and chemically. Thus, during etching of the side portion of a patterned metal film, the DLC film is hardly etched, and a overhung portion of the DLC Film and a cavity can be easily formed. As a result, electric capacitance between wirings can be further reduced compared with the case using the SiN film as a first insulating film.
Furthermore, instead of the above SiN and DLC films, an SiO2 film may be used as a first insulating film. A process for forming the SiO2 film has been well established and, accordingly, the SiO2 film as a first insulating film of the present invention can be easily formed by using an existing film forming apparatus. Since the SiO2 film contains little moisture during film formation, as compared with the SiN film, wirings formed under it are hardly affected by moisture. Moreover, the dielectric constant of the SiO2 film is about 4.0, which is smaller than the dielectric constant (about 7.0) of the SiN film. As a result, the use of the SiO2 film as a first insulating film enables the dielectric constant of the entire film to be smaller than that of the case using the SiN film as a first insulating film.